1. Field of the Invention
The present invention relates generally to gas phase calibration standards for the calibration of analytical instruments and more particularly to field preparation of low concentration gas phase calibration standards.
2. Description of Related Art
Laboratory or commercial preparation of low concentration standards is known in the art. For example, a dilution system using mass flow controllers to effect a precise proportion of two gases is shown in FIG. 1. This system comprises a source of pure gas, comprised of a container of gas (not shown) and a purifier 100, whose flow is controlled by a first thermal mass flow control device ("MFC") 102. It further comprises a source of a "standard mixture" 104 (a mixture of pure gas with a known quantity of some impurity) whose flow is controlled by a second MFC 106. The two flows from the MFCs 102 and 106 are combined in a mixing volume 108 such that the ratio of the flows, as regulated by the MFCs 102 and 106, determines the dilution ratio. An excess flow venting device 110 (e.g. a pressure regulator as shown in FIG. 1) provides a means of controlling the flow of the mixed gases to the ultimate destination. This system requires electric power to operate and needs time to reach thermal equilibrium. It is bulky and expensive and not conducive to work in the field. Other commercial systems which operate by combining flows through complicated arrays of capillaries are difficult to manufacture and also very expensive.
In practice, low concentration standards are frequently prepared well in advance of their use and stored in gas cylinders. This pre-mixing to produce a diluted gas of fixed concentration can become a problem when the desired low concentration impurities (typically organic compounds) are suspected of interacting destructively with the internal surfaces of the storage vessel.
For many applications where gas phase calibration standards (consisting of an accurately known concentration of one or more impurities in a balance of pure gas) are required, it is desirable to dilute a standard at high concentration with a pure gas to obtain the desired concentration. Such an approach would avoid many of the problems associated with the storage of pre-mixed standards and provide multiple concentration levels.
Flow of gases through an orifice under "critical" conditions is a relatively well understood phenomenon described in standard texts on fluid dynamics. Several of its features have been employed in a number of different applications.
For example, U.S. Pat. No. 4,878,510 to Kasper et al. discloses a method which avoids the formation of droplets by condensation of vapors during the expansion of a highly compressed gas through a critical orifice. Co-pending application Ser. No. 107,177 to Wen et al. assigned to the assignee of the present invention discloses a method for quantifying the concentration of condensible vapors in a carrier gas by using the minimum pressure drop across a critical orifice to cause droplet formation and then detecting the number of droplets for comparison with the volume of gas which has passed through the critical orifice.
Several applications have employed critical orifices as flow control devices. Soviet Union Patent No. 1,334,109 to Isakov et al. relates to a method to control liquid flow by first vaporizing a liquid, passing the vapor through a critical orifice and then recondensing the liquid. U.S. Pat. No. 4,842,827 to Graf et al. describes the use of critical orifices to control the flow of gases to a vacuum system. A high purity "fixed" gas is used to measure the orifice size which is then used to calculate the flow of reactive or toxic gases through the orifice.
Co-pending application Ser. No. 437,615 now U.S. Pat. No. 5,054,309 and Ser. No. 437,623 now U.S. Pat. No. 5,157,957 to Mettes et al. assigned to the assignee of the present invention disclose a process and apparatus to generate low concentration calibration mixtures for testing and/or calibrating analytical instruments such as an atmospheric pressure ionization mass spectrometer. The Mettes inventions employ critical orifices to simultaneously control the flow of multiple gas streams to achieve a known dilution. However, these inventions relate to a system which is large and complicated and is designed for laboratory use. Still further, the system requires multiple mixing steps with more than two gas sources and a moisture generator.